Science & Nature

Why is the sky blue and the sunset red?

Zolvelo Team3 min read

White light is really a mix of colours

Light from the Sun looks white to us, but it contains every colour from violet to red. What separates these colours is their wavelength: violet and blue have short wavelengths (about 400-450 nanometres), while orange and red have long ones (600-700 nanometres). When a prism or a raindrop pulls this mix apart, we see a rainbow.

Why does air scatter more blue?

The atmosphere is made mostly of nitrogen and oxygen molecules. These molecules are much smaller than the wavelength of light. Light that hits such tiny particles is scattered in every direction. This is called Rayleigh scattering, after the British physicist Lord Rayleigh, who explained it in 1871.

The rule is simple, but its effect is big: as the wavelength gets shorter, scattering increases very quickly. Blue light is scattered roughly five to six times more than red. The light that reaches our eyes from every direction except straight from the Sun is this scattered blue light.

So why isn’t the sky violet?

Violet light has an even shorter wavelength than blue, so it is scattered even more. Yet we don’t see a violet sky. There are two reasons: sunlight contains less violet than blue, and our eyes are far less sensitive to violet than to blue. Our brain reads the mix that arrives as light blue.

A pale blue, translucent glass stone on a white surface. There is an orange patch of light inside its shadow.
This translucent glass looks blue because the tiny particles inside it scatter blue light; the light that passes through and falls into the shadow turns orange. The sky does the same thing on a huge scale. Photo: optick, CC BY-SA 2.0.

Why do the colours change at sunset?

When the Sun is high, its light takes a short path through the atmosphere. As it nears the horizon, the light has to cross a much thicker layer of air sideways. At the horizon this path is about 38 times longer than when the Sun is directly overhead. On this long journey most of the blue and green is scattered away; orange and red tones are what reach our eyes.

Dust, smoke or volcanic ash in the air can strengthen this effect. After the 1883 eruption of Krakatoa, unusually bright, fiery sunsets were seen in many parts of the world for months.

Why are clouds white?

The water droplets that make up clouds are much bigger than air molecules. Large particles scatter all colours almost equally. Because the colours mix back together, we see clouds as white or grey.

On Mars it works the other way round

Mars has a very thin atmosphere with fine dust grains hanging in the air. By day this dust gives the sky a pale, yellowish tint. At sunset, though, the area around the Sun looks bluish. NASA’s Curiosity rover photographed this blue sunset in 2015.

Sources

  1. NASA Space Place, “Why Is the Sky Blue?”
  2. Met Office, “Why the sky is blue and sunsets red”
  3. NASA Science, “Sunset in Mars’ Gale Crater” (2015)

This article was prepared with AI assistance and reviewed by an editor. Spotted a mistake? Let us know.